1. Field of the Invention
The present invention relates to an optical transmitter.
2. Related Background Arts
Digital coherent transmission technology, which utilizes phase information of light wave, has been implemented for increasing capacity of communication network. The digital coherent transmission technology applies modulation systems like DP-QPSK (Dual Polarization—Quadrature Phase Shift Keying) and DP-16QAM (Dual Polarization—16 Quadrature Amplitude Modulation) to enable a transmission speed beyond 100 Gbps per wavelength. The QPSK system utilizes quadrature phases of an optical carrier wave for modulation and demodulation, and the QAM system further utilizes multi-levels in amplitude of the optical carrier wave for higher speed modulation and demodulation. The QPSK system and 16QAM system respectively treats four and sixteen symbols in constellation diagram thereof. The DP multiplexing technique utilizes two polarized optical carrier waves, which are orthogonal to each other, for multiplexing the modulated optical signals by the QPSK system or QAM system.
Some optical transceivers (coherent optical transceiver) which perform such phase modulations/demodulations and the polarization multiplexing are being developed. Each coherent optical transceiver mostly includes a transmission part (optical transmitter) and reception part (optical receiver). The transmission part includes a multi-level phase modulator and a driving circuit (driver). The multi-level phase modulator has a nested structure constituted by a plurality of Mach-Zhender modulators. The driving circuit generates driving signals to drive the Mach-Zhender modulators for the QPSK system or QAM system.